The present invention relates to a projection type image display device for making image display using a laser as a light source.
A projection type image display device, that is, a so-called projector device is known as an image display device capable of large screen display.
A projection type image display device that displays images by using a laser as a light source and scanning a laser beam has been considered. The projection type image display device has advantages of providing ease of modulation with an image signal and good color reproducibility, and ensuring sufficient brightness, for example (see Patent Document 1, for example).
Laser light is collimated light emitted from a very small light emission point and having a small angle of divergence. Therefore, if the light directly enters the eye, the light concentrates as a small spot on the retina of the eye. When its laser power is high, the light causes a burn on the retina of the eye.
Thus, an international laser safety standard IEC 60825 series and internal standards of each country based on the international laser safety standard IEC 60825 series for devices using laser light provide stringent safety standards for manufacturers and users of laser devices. IEC 60825-1, for example, makes classification into classes 1 to 4 according to magnitude of laser power and provides guidelines to be followed by manufacturers and users for each class.
It is confirmed that an image display device scanning a laser beam over a screen is safe even with a laser of a considerably high intensity (laser class 3B or below), because laser light hits the screen and is then reflected and diffused in all directions.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 3-65916
However, if the laser beam before being applied to the screen is directly looked at accidentally and an amount of light of the laser beam is large, the retina of the eye may be burned.
The risk is related to energy density (W/m2) of the laser light, exposure duration, and size of an image point on the retina.
The higher the energy density (W/m2) of the laser light, the longer the exposure duration, and the smaller the size of the image point on the retina, the greater the risk of a burn.
An international standard set by an international laser safety commission defines maximum permissible exposure (MPE) of the retina when laser light directly enters the eye. The maximum permissible exposure (MPE) represents a reference value based on various experiments and assessments by the commission. The value is at or below that no damage is caused to the eye of a person exposed to laser light (see the commission's international standard IEC60825-1 amendment 2 p31 Table 6).
FIG. 6 shows a relation between exposure duration and permissible laser irradiation intensity (energy density [W/m2]) for a fixed image point on the retina. The point is of a size of a minimum perceived image point or smaller (the most stringent condition of C6=1 in the above table; when a laser light source is a smallest object point) on the basis of a numerical value in a region of visible light (wavelengths of 400 nm to 700 nm) of the maximum permissible exposure defined in the above international standard.
FIG. 6 indicates that when the exposure duration becomes shorter than 0.05 seconds, the permissible laser light intensity is sharply increased, whereas in a region of longer exposure duration than 0.05 seconds, the permissible laser light intensity is not decreased very much even when the exposure duration is increased.
That is, as a tendency, the risk of causing damage to the eye by laser light is reduced as the laser exposure duration is shortened, while as the exposure duration becomes longer than about 0.05 seconds, decrease in the permissible laser light intensity is reduced.
This indicates that when the exposure duration is shorter than about 0.05 seconds (a one-shot laser light irradiation), safety can be enhanced by minimizing a time for which the eye can be irradiated, that is, by shorter pulse irradiation.
When the exposure duration becomes longer than about 0.05 seconds (in cases of a continuous-wave laser, a continuous pulse oscillation laser, and periodic scanning of laser light, for example), safety can be enhanced more effectively by decreasing laser light intensity (W/m2) per unit time and per unit area rather than shortening a sum total of exposure duration.
From the above consideration, considering safety when audience directly looks at projected laser light, a device that makes image display by periodically scanning laser light is required to decrease laser light intensity (W/m2) as much as possible.
Conventional devices that make color display by scanning laser light are not designed from this point of view.
FIG. 7 is a schematic diagram of a conventional laser light scanning color image display device.
Pieces of laser light of three primary colors, that is, red, green, and blue are modulated by an image signal. These pieces of laser light of the three colors are multiplexed into one laser beam. The laser beam is passed through a projecting lens and scanned by a scanning mirror to display an image on a screen.
In the case of this device, when the laser beam enters the eye directly (rather than enter the eye as light diffused and reflected from the screen), a total amount of laser beam output of red, green, and blue enters the eye simultaneously.
This system is desirable from a viewpoint of ease of color shift adjustment for the three colors on the screen.
However, as shown by the above consideration, considering laser safety for the eye, the system cannot increase laser light intensity (W/m2). Consequently the system cannot display brighter images.